Electrolytic production of fluorine



Patented 12,- 1932 UNITED STATES 'FREDIlNHAGEN, F GREIFSWLLD, GERMANY ELECTROLYTIC PRDDU CTIQIE 0F FLUORINE No Drawing. Application filed July ll, i928, Serial No. 377,808, and in Germany luly 17,1928.

The present invention relates to the electrolytic production of fluorine.

It is known that fluorine can be produced electrolytipall from fused alkali metal hifluorides or rom solutions of alkali metal fluorides in hydrogen fluoride. In thebeginning of the process oxygen is first obtained, since the electrolyte always contains some water. At the sametime the water present ll therein causes the electrolysis to proceed in a very irregular manner and also the voltage varies considerably. Attempts have been madeto remove the traces of water present b the electrolysis itself, but this requires el ectrolysis for many hours, whereby the variations of the voltageare reduced but not completely removed.

l ave now iound that fluorine can be produced directly in a uniform manner and without the undersirable generation of oxygen by subjecting the alkali metal bifluorides prior to their serving as the electrolyte in the electrolysis vessel itself to a severe drying operation, preferably by passing a current of (1 air or other dry inert gas through the su ciently cemminuted salt at an elevated temperature, but below the meltin point of the salt, until the hydrogen fluori e givenofi by the alkali metal bifluoride when occasionally so heated to decomposition, has a specific conductivity; after condensation, at 0 C. of less than 0.2 reciprocal ohm. Preferably the electrolysis vessel is tight to the air so that no watervapour can enter the electrolytic chamher. For this purpose every opening of the vessel must be avoided and an insulating tightening between the anode and the metallic diaphragm and another one between the vessel serving as the cathode and the diaphragm or the anode must be rovided. In. case the diaphragm consists o cop rit is not absolutely necessary that the tlghtening between 5Q still: is best obtained by connecting the dryhe same result is obtained when the elecing chamber with the electrolysis vessel, so that the dried salt can be brought from the drying chamber to the electrolysis vessel withoutcomin into contact with the atmosphere. Accor ingly the drying chamber is to be regarded as part of the electrolysis system. i

Since in the course of the electrolysis the electrolyte becomes poorer in hydrogen fluoride whereby the solidification point of the electrolyte, the internal electric resistance andaccordingly the decomposing voltage are increased, it is necessary for a continuous operationto replace the hydrogen fluoride in the electrolyte solution or melt. If hydroto gen fluoride prepared in the usual manner and containing more or less water is used for this purpose, the disengagement of fluorine is decreased and unfavourably influenced in the aforesaid manner and the anodic volt- 7 age is increased- In order to continue the electrolysis without trouble, the hydrogen fluoride in the electrolyte is replaced by such hydrogen fluoride as contains only an ex tremely small amount of water, namely such. 7 that the condensed hydrogen fluoride has a specific conductivity at 0 C. of less than 0.02 reciprocal ohm.

The-following example will further illustrate the nature of this invention which, how- 50 -ever, is out thereto.

Example Potassium 'bifluoride dried in the usual manner is subjected to electrolysis in an elecs5 tr'olyzing vessel,'between a copper cathode and a graphite anode. Electrolysis takes place very irregularlyr The voltage varies between 10 and more than 30 volts whereby the current intensity decreases with increasi g voltage. With an increasing current dhnsity, the variations of the voltage occur more readily and with a type of apparatus used for these experiments it was not possible to increase the current density to substantialo5 1y more than 0.1 ampere 'per square centimeter. The said variations of the voltage occur very often at the beginning of the electrolysis but less often after a longer time, but even after 15 hours of operation they are not entirely avoided and the disengagement of fluorine is very irregular.

In accordance with the present invention the same salt is dried within the electrolysis vessel by means of a current of dry air passed through at a temperature below the melting point of the salt. When the salt thus treated is electrolyzed, the electrolysis proceeds in a I very uniform manner and without variations of the voltage. When operating with a tension below 10 volts the current density can be increased, in the case of the apparatus above referred to. to more than 0.3 ampere per square centimeter without the voltage being substantially increased or the electrolysis becoming irregular.

After along time of operation the melt becomes lower in hydrogen fluoride and'in order to continue the electrolysis without interruption, hydrogen fluoride is added. When this is carried out by producing hydrogen fluoride from potassium bifluoride and absorbing it in themelt, very great variations of voltage and current density take place in electrolysis, and it was found that the water content of the added hydrogenv fluoride is the cause. The said variations of current density and tension are'not observed, if care is taken that the added hydrogen fluoride is practically free from water, that is that it has the specific conductivity indicated above. When adding to a used up melt hydrogen fluorideof this kind the electrolysis can be continued without variations.

What I claim is 1. The process of producing fluorine which comprises passing a current of a dry inert gas through an alkali metal bifluoride in the electrolysis, system at an elevated temperature, but below the melting point of the salt, until the hydrogen fluoride given off by the salt when occasionally heated to decomposition has-aspecific conductivity at 0 C. of less than 0.02 reciprocal ohm, and carryingout electrolysis with the salt in a liquid condition while avoidin 'access of moisture;

2. The process of prod iicing fluorine which comprises passing a current of a dry inert gas throu h an alkali .metal bifiuoride in the electrolysis vessel at an elevated temperature, but below the melting pointof the salt until the hydrogen fluoride givenofi by the salt when occasionall heated to decom osition has a specific conductivity at 0 C, 0 less than 0.02 reciprocal ohm, carrying out electrol sis with the salt in a liquefied condition trolysis system at an elevated temperature, but below the melting point of the salt, until the hydrogen fluoride given off by the salt when occasionally heated to decomposition has a specific conductivity at 0 C. of less than 0.02 reciprocal ohm, and carrying out electrolysis with the salt in avliquid condition while avoiding access of moisture.

4. The process of producing fluorine which comprises passing a current of a dry inert gas through potassium bifluoride in the electrolysis vessel at an elevated temperature, but below the melting point of the salt until vthe hydrogen fluoride. given off by the salt when occasionally heated to decompositionhas a specific conductivity at 0 C. of less than 0.02 reciprocal ohm, carrying out electrolysis with the salt in a liquefied condition while avoiding access of .moisture, adding hydrogen fluoride free from water and having a specific conductivity at 0 C. of not more than 0.02 reciprocal ohm to the electrolyte, in order to maintain its liquid condition and continuing electrolysis.

In testimony whereof I havehereunto set my hand.

KARL FREDENHAGEN.

' whi e avoiding access of moisture, adding hydrogen fluoride free from water and having a specific-conductivity at 0 C., of not more than 0.02 reciprocal ohm to the electro- ,7 l te, in order to' maintain its liquid conditlOIl and continuin electrolysis.

3. The processo producing fluorine which comprises asslng a current of a dry inert 'gas throng potassium bifluoride in the elec- 

